Create Offline NameNode recovery tool
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Key: HDFS-3004
URL: https://issues.apache.org/jira/browse/HDFS-3004
Project: Hadoop HDFS
Issue Type: New Feature
Components: tools
Reporter: Colin Patrick McCabe
Assignee: Colin Patrick McCabe
We've been talking about creating a tool which can process NameNode edit logs
and image files offline.
This tool would be similar to a fsck for a conventional filesystem. It would
detect inconsistencies and malformed data. In cases where it was possible, and
the operator asked for it, it would try to correct the inconsistency.
It's probably better to call this "nameNodeRecovery" or similar, rather than
"fsck," since we already have a separate and unrelated mechanism which we refer
to as fsck.
The use case here is that the NameNode data is corrupt for some reason, and we
want to fix it. Obviously, we would prefer never to get in this case. In a
perfect world, we never would. However, bad data on disk can happen from time
to time, because of hardware errors or misconfigurations. In the past we have
had to correct it manually, which is time-consuming and which can result in
downtime.
I would like to reuse as much code as possible from the NameNode in this tool.
Hopefully, the effort that is spent developing this will also make the NameNode
editLog and image processing even more robust than it already is.
Another approach that we have discussed is NOT having an offline tool, but just
having a switch supplied to the NameNode, like "—auto-fix" or "—force-fix". In
that case, the NameNode would attempt to "guess" when data was missing or
incomplete in the EditLog or Image-- rather than aborting as it does now. Like
the proposed fsck tool, this switch could be used to get users back on their
feet quickly after a problem developed. I am not in favor of this approach,
because there is a danger that users could supply this flag in cases where it
is not appropriate. This risk does not exist for an offline fsck tool, since
it would have to be run explicitly. However, I wanted to mention this proposal
here for completeness.
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